Effects of calcium ions on solubility and aggregation behavior of an anionic sulfonate gemini surfactant in aqueous solutions.

Effects of calcium ions on the solubility and aggregation behavior of an anionic sulfonate gemini surfactant 1,3-bis(N-dodecyl-N-propylsulfonate sodium)-propane (12-3-12(SO(3))(2)) have been studied in aqueous solution. Compared with single-chain surfactant sodium dodecylsulfate, 12-3-12(SO(3))(2) shows much better performance to the hardness tolerance with calcium ions. Moreover aggregates of the Ca(2+)/12-3-12(SO(3))(2) complexes in clear solutions influence the morphologies of the precipitates. At 12-3-12(SO(3))(2) concentrations lower than 1.5 mM, the small spherical micelles of Ca(2+)/12-3-12(SO(3))(2) in clear solutions generate precipitates of solid particles owing to complexation of surfactant monomers with Ca(2+). At 12-3-12(SO(3))(2) concentrations higher than 1.5mM, the Ca(2+)/12-3-12(SO(3))(2) complexes transform into large compact spherical aggregates and then into long wormlike micelles. These large aggregates are well dispersed in aqueous solutions and efficiently complex calcium ions. In particular, long wormlike micelles are entangled with each other at 100.0 mM CaCl(2) and 100.0 mM 12-3-12(SO(3))(2) exhibiting viscoelastic properties. In addition, the stacking of long wormlike micelles produces precipitates with ordered fibrillar structures. This work reveals that such anionic sulfonate gemini surfactants are better candidates than single-chain surfactants in applications with high hardness levels, and the ordered aggregate structures may have potential applications in materials science.